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voltage regulator question

Discussion in 'Electronic Basics' started by tempus fugit, Dec 1, 2003.

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  1. tempus fugit

    tempus fugit Guest

    Hey all;


    My understanding of a voltage regulator is that it is supposed to deliver
    (in this case) 15v regardless of the current draw (within spec of course).
    However, if I drop a 2K resistor to ground on my homebrew PS, I notice that
    the voltage drops about 10-15 mV. Or am I off base - is it only supposed to
    deliver 15v regardless of line voltage (assuming the line voltage is above
    the dropout voltage of the regulator)?

    Put another way, if I'm looking at the voltages on a piece of gear (could be
    anything) that's using say, a regulated +15v and -15v, will the voltages at
    the regulator read +15 or -15v with the unit on? I.e., if the unit is
    drawing some current, but not beyond what the regulator is spec'd for, will
    there be a corresponding voltage drop, or is the regulator supposed to keep
    the voltage at +15v irregardless?

    Thanks
     
  2. CFoley1064

    CFoley1064 Guest

    Hey all;
    Hey, "tempus". How about some more information? What does "home brew" mean?
    Did you use a 7815, an LM723, or something else? Linear or switching
    regulator? A little more information might give someone enough information to
    give you an informed answer.

    The term you're looking for is "load regulation", and is usually defined in
    terms of percent (of nominal voltage) from no load to rated load, or from
    minimum to maximum load. It states that the regulated voltage will not vary
    more than that much over the range of loading specified.

    For a small switching power supply, 15 mv drop (0.1%) from no load to 7 mA
    wouldn't be unusual at all -- actually, I'd expect at least that much. For a
    linear power supply, it might mean something's wrong, but not necessarily. You
    might want to try different loads on the power supply, and seeing if it's
    relatively stable over a wider load range. If the load regulation stabilizes
    and remains less than rated spec, you're probably OK. If not, check your
    wiring and layout first.

    Good luck.
    Chris
     
  3. John Larkin

    John Larkin Guest


    I've seen (and built) several switchers that had negative regulation:
    the higher the load current, the higher the output voltage. I've never
    really analyzed why.

    John
     
  4. tempus fugit

    tempus fugit Guest

    Sorry Chris - it is a variable dual polarity supply using a 317 and a 337
     
  5. cpemma

    cpemma Guest

    If you've used high-value resistors in the R1/R2 control chain you'll see
    that drop, check the datasheet for minimum load current.
     
  6. default

    default Guest

    As someone has already said, you don't mention the specifications for
    load regulation -

    One other thing is where you measure the voltage drop - at the point
    of regulation, or at the end of a few feet of wire along with the load
    resistor.

    If the regulator is "remote sensing" it would boost/cut the output
    voltage from the regulator so it would remain within the load
    regulation spec at the point of use. (compensates for drop in the
    wiring).

    The word you want to use is "regardless."
     
  7. tempus fugit

    tempus fugit Guest

    OK I tried dropping a 100 ohm resistor to ground and got a voltage drop of
    about 2.5v this time (i.e., the voltage went from 15v to 12.5v). The
    regulator in question is a 317 (I get the same results with a 337). Both of
    these specify a 0.1% load regulation. Obviously, the 2.5v drop is a tad
    higher than 0.1%.
    Am I testing this properly, or is this what I should be seeing?

    Thanks
     
  8. default

    default Guest

    Right on! Well outside of specification for the part (assuming the
    wire resistance is low and you measured at the regulator).

    Given that the wire length and connections are good:

    The logical explanations are bad part(s) or too high ripple (not
    enough filtering) or poor transformer regulation.

    What is the input to the regulator under load? It has to be 4 volts
    higher than the output voltage. That is four volts to the "valley" of
    the ripple. (4 volts is typical for a LM317)

    Some generalizations: Transformer regulation is usually (but not
    always) better than 10% no load to full load. Big heavy transformers
    usually have better regulation (at low currents especially).

    A good rule of thumb for filter caps is 1,000 micro farads per 1/2 amp
    of current, for about 10% ripple. (electrolytic caps can be specified
    at -10% to +125% so you might check that spec as well)

    You must be overlooking something.
     
  9. default

    default Guest

  10. tempus fugit

    tempus fugit Guest

    Thanks Default. I had a few minutes to check some things out tonite. The
    input to the regulator is about 18v, which is cutting it a little close for
    15v, but I get similar results when it is set to 5v. I scoped the input to
    the regulator; there is no ripple here (I'm using 4700uF caps).

    I'll double check the wiring and make sure everything's OK there too.

    Thanks again
     
  11. tempus fugit

    tempus fugit Guest

    I found the problem. I had put 2 8 ohm resistors in series with the output
    to protect from accidental shorts. This would keep the maximum current draw
    at a little less then an amp (cutting it close I know). When I tested it
    without the resistors, it gave me a consistent output voltage.

    Of course, I now have no short circuit protection. Any ideas for something
    simple to provide that? Also, why did the 16 ohms in series with the output
    stop the regulator from regulating?
    Thanks
     
  12. default

    default Guest

    Well that would certainly do it. Putting 16 ohms in series with the
    output drops voltage.

    The water pipe analogy: pipe (resistor) is too small for the flow
    (current) and pressure builds up on the inlet side and drops on the
    outlet side (voltage drop across the resistor).

    If I'm not mistaken, the LM317 adjustable three terminal regulator has
    built in over current protection at something like 1.5 amps (will
    protect itself and the transformer if the transformer can output 1.5
    amps safely. Voltage drops as current tries to increase beyond its
    design value. (load resistance decreases)

    The LM317 will get warm or hot while limiting and it also includes
    thermal protection, so the output current maximum will come down as
    the part gets hot.

    If you want some other current limit (less than the 1.5 amps the part
    is good for) you'd have to add an external resistor to sense the
    current and shut down the regulator - but then the design gets tricky
    and you loose the ease of application of the 3 terminal regulator.

    I generally will use a TO92 size regulator if I want to keep the
    current low; they limit at 100 milliamps. If I need something else I
    go for the LM723/UA723 (14 pin dip package) and external pass
    transistors - that allows a current limit to be set with an external
    resistor while keeping the resistor inside the control loop (doesn't
    change the regulation - the resistor only need drop .6 volts to limit
    the current)

    The databook (National Semiconductor Voltage Regulator Handbook) shows
    a current limited battery charger. That takes an additional
    transistor and three resistors and one resistor is in the ground leg
    in series with the load (battery being charged). They show an
    adjustable current regulator using a 3 terminal part (LM338 - 5 amp
    regulator so the LM317 would also work) to achieve adjustment they add
    another LM117 to handle the current regulation function.

    You don't say that you need the current to be adjustable just want
    short protection - the LM317 already does that.

    The National Semiconductor web site has the application notes on it
    and they will ship you a CD of their web site with all the parts they
    make for a nominal shipping cost or free. The CD is searchable just
    like the web site if you use a browser to open it. The regulator
    handbook was also free, but published in 1982 and is probably out of
    print now.
     
  13. tempus fugit

    tempus fugit Guest

    "> Well that would certainly do it. Putting 16 ohms in series with the
    OK, but I figured that there would already be a certain resistance between
    the output and ground (say, 100 ohms) and adding a little more wouldn't
    change things much, particularly when it was much smaller than the circuit's
    resistance (i.e., If it was 100 ohms at 10v, the current draw would be 100
    mA, so it would be a little less at 116 ohms). Also, the voltage drop varied
    with the current draw - it was pretty major when drawing 100mA, but only a
    few 100 mv whn drawing 30 or 40 mA. Why would that be?

    That's good news. I'll check the datasheet and see if it does.
    No, I don't really need adjustable current, just wanted something to prevent
    me from having to replace the regulator IC every time I accidentally touched
    the hot out to ground.
    Thanks again
     
  14. default

    default Guest

    I don't know why that would be, but the first thing I'd look at would
    be the input voltage with a meter or better yet a scope to see what it
    looked like. You have to maintain the 4 volt differential between in
    and out or the part drops out of regulation. The meter averages the
    voltage so it won't tell you if there is too much ripple on the input.

    "Low Drop Out" regulators will run with ~.6 to 1.0 volt differential.
    (makes them more efficient and less heat to dissipate - if the input
    voltage is lower with respect to the output)

    You may have a problem with too much ripple, bad connections, or just
    a ground loop causing an oscillation. The three terminal regs are
    easy to apply, but they can be made to oscillate if there's a ground
    loop or too much inductance on the output.

    Without a scope you won't see some of the things that can go wrong.

    Oscillations are not common, at least not with a good layout . . .
    Most designers probably never encounter an oscillation in a 317. A
    small output cap can do wonders if the load is inductive and causing
    an oscillation. For a layout problem, the layout has to be corrected.
    Three terminal regs are as close to bullet proof as any part is. You
    can destroy them by putting a reverse current through them by
    connecting the input backwards, or having a large capacitor on the
    output and dropping the input voltage suddenly (like discharging the
    filter cap with a screw driver) Once they are wired correctly and in
    use they last a long time.
     
  15. tempus fugit

    tempus fugit Guest

    Thanks again Default.

    I checked the data sheet, and it says that the typical current limit on the
    TO220 (which is what I'm using) is 2.2A. Wouldn't that destroy the regulator
    (which is spec'd for 1.5A) in a short circuit situation? Or am I looking at
    the wrong spec (or interpreting it wrong)?

    Thanks again.
     
  16. default

    default Guest

    The part is spec'd with a minimum of 1.5, typical of 2.2, and maximum
    of 3.7.

    The one you get will fall in that range. If you are a commercial
    designer you use the minimum current, because they guarantee the part
    to work at 1.5, the other currents are possible and most of the parts
    they turn out are 2.2.

    If you are in a position where you just bought 100,000 three terminal
    regulators for a product - they don't want you sending them back if
    they only output 1.5 - and they don't want to select parts for a
    particular user . . .

    That doesn't mean the regulator will be destroyed at 2.2 amps out.
    With a good heat sink it should put out 2.2 amps or even 3.7 without
    complaining. But if it only puts out 1.5 before it current limits -
    it is still a good part.

    The parts tend to get better with manufacturing experience, so it may
    be that very few will output only 1.5 these days - but if you are
    committing large amounts of money to a product, you would be wise to
    observe the lower limit or call National and see what they are willing
    to do for you.

    From your hobiest/experimentor perspective, you don't have to worry
    about the part, it is protected from over current and shorts.

    You still need to worry about the power source. If you have a
    transformer supplying power for the device it must be able to source
    3.7 amps to the regulator without burning up. (that could actually be
    higher - like 5 amps - depending on how the transformer is specified)

    For arguments sake, you have a transformer that will put out 12 volts,
    at 3 amps. That is 12 volts RMS AC voltage at 5 amperes. Use a full
    wave bridge rectifier to filter it and you might have 17 volts of DC
    (1.4142 * the RMS voltage, give or take a little for transformer
    regulation and rectifier losses)

    That doesn't mean you get to suck 17 volts at 3 amps from the filter
    cap side of things. 3 amps/17 volts at the filter is close to 4.25
    /12 at the transformer.

    If your only concern is that the regulator and transformer survive a
    temporary short, don't sweat it. If you are building something like a
    battery charger where the over-current condition can last for hours it
    is a different matter.

    I learned that lesson a long time ago. Wanted some tunes at work and
    built an amplifier, but didn't consider the difference in AC and DC
    amps. My power transformer was sealed in a steel case and potted in
    tar. I fell asleep one night listening to music in the darkened shop.
    The transformer exploded and woke me up.

    I also learned something from this discussion. I was looking at class
    A audio amps and it occurred to me that one could be made by using a
    single output transistor working against a constant current source
    (for a push-pull output). That was a year ago - but I didn't try it.
    Looking at the old regulator book I saw where they actually do it.
    Neat idea.
     
  17. default

    default Guest

    made a mistake there

    For arguments sake, you have a transformer that will put out 12 volts,
    at 3 amps. That is 12 volts RMS AC voltage at 5 amperes. Use a full
    wave bridge rectifier to filter it and you might have 17 volts of DC
    (1.4142 * the RMS voltage, give or take a little for transformer
    regulation and rectifier losses)

    That is 12 volts RMS AC voltage at 3 amperes (not 5)
     
  18. cpemma

    cpemma Guest

    The L200 adjustable regulator is a 5-pin 2A device, very easy to fit current
    limiting (1 extra resistor) if you need to protect the transformer.

    On your voltage drop, you produced a simple potential divider - 15v over 16R
    + 200R gives 15*200/216 =13.9 volts over the 200R.
     
  19. tempus fugit

    tempus fugit Guest

    I wasn't sure if there was a difference between AC and DC amps. So do you
    multiply it by 1.414 just like RMS voltage? (3A*1.414=4.25A)?

    That's really my only concern; I use it as a bench supply for prototyping
    or repair. I'm sure that I've been careless (stupid?) enough to actually
    hook the supply up to a circuit that has a short in it, or (more
    embarrassing) plugged the hot and ground into the same connection on a
    protoboard. So I guess in this situation the overcurrent situation could
    last a minute or so before I caught it. As long as it will withstand
    something like this I'm happy.
    Cool. I'm glad I'm helping you too.
     
  20. default

    default Guest

    I wasn't sure if there was a difference between AC and DC amps. So do you
    Yes, except you have it backwards - as the voltage goes up with
    rectification and filtering the current allowed on the output goes
    down for the same power level at the transformer.

    Multiply by the reciprocal of 1.414 - .707
     
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